It is known in a prior art to provide an SCR filter made up of a filter and an SCR catalyst (selective catalytic reduction NOx catalyst) supported on the filter in an exhaust passage of an internal combustion engine. The SCR catalyst has the capability of reducing NOx in the exhaust gas using ammonia as a reducing agent. The filter has the function of trapping particulate matter (which will be hereinafter referred to as PM) in the exhaust gas.
Patent Literature 1 discloses a technology pertaining to an exhaust gas purification system including an SCR catalyst provided in an exhaust passage of an internal combustion engine. Patent Literature 1 teaches to estimate the ammonia adsorption amount defined as the amount of ammonia adsorbed in the SCR catalyst and to adjust the quantity of reducing agent supplied to the SCR catalyst on the basis of the difference between the estimated ammonia adsorption amount and a target adsorption amount. By this technology, it is possible to reduce the quantity of ammonia flowing out of the SCR catalyst while keeping the NOx removal rate of the SCR catalyst (i.e. the rate of the quantity of NOx reduced by the SCR catalyst to the quantity of NOx flowing into the SCR catalyst) high.
Patent Literature 2 discloses a technology pertaining to an exhaust gas purification system including an SCR catalyst provided in an exhaust passage of an internal combustion engine at a location downstream of a filter. Patent Literature 2 teaches to correct the quantity of reducing agent supplied to the SCR catalyst taking account of the amount of PM deposited in the filter.
Non-patent Literature 1 discloses the fact that the quantity of ammonia adsorbed in an SCR catalyst supported on an SCR filter tends to increase as the amount of PM deposited in the SCR filter increases.